Conchostracans of the Middle-Late Jurassic Daohugou and Linglongta beds in NE China

The Yanliao biota is widely distributed in western Liaoning, northern Hebei, and the areas adjacent to eastern Inner Mongolia. The biota is excavated from two typical fossil-bearing beds: the Daohugou fossil-bearing beds in Daohugou, Jiangzhangzi, and Chentaizi, Wuhua Town, Ningcheng County, Inner Mongolia; and the Linglongta fossil-bearing beds in Daxishan, Linglongta Town, Jianchang County, Liaoning Province and Bawanggou, Mutoudeng Town, Qinglong County, Hebei Province. Previously, it was thought that the Daohugou and Linglongta beds are correlatable in age and fossil content and that the conchostracans from both the Daohugou and Linglongta beds could all be attributed to the genus Euestheria. However, clear punctae but no reticulations are observed on all conchostracans collected from the Daohugou beds, which excludes them from Euestheria. In this study, only one species, Triglypta haifanggouensis n. comb., is recognized based on detailed investigations of the ornamentations of hundreds of specimens from the Daohugou beds. In addition to the recently reported Tianzhuestheria jianchangensis, another species, Liaoxiestheria linglongtaensis n. gen. n. sp., is identified in this study based on a large number of specimens from the Linglongta beds in Daxishan. Isotope analysis has shown that the Daohugou and Linglongta beds are approximately 165 Ma and 160 Ma old, respectively. Therefore, the fossils from the Daohugou beds represent an early assemblage and the fossils from the Linglongta beds represent a late assemblage of the Yanliao biota. This study indicates that the conchostracan assemblage of the Daohugou beds is a typical Triglypta-Qaidamestheria assemblage, which is widely distributed in the Middle Jurassic of North China, and the conchostracan assemblage of the Linglongta beds represents the late, succeeding Triglypta-Qaidamestheria assemblage.